Wow.
So much input. This is great, thanks.

Let's see if I have this right:
Coolant sensor drops resistance at 105 degrees, which kicks (relay?) N22, which in turn kicks relay K9 to turn on the fan.

If I am understanding this right, one possibility is that the circuit has some 'static' resistance, such as a corroded wire, terminal. or somesuch. If true, the resistance may remain too great even when the sensor drops to exerting whatever ohms at 110 degrees, which is the highest temp I tried.

Jumping the sensor, (theoretically Zero resistance) starts the fan (just one fan in my case), but perhaps this is not really Zero resistance, but a small enough R value to trip N22.

Unless the harness has conductivity problems, it would seem that I need to replace Relay N22. Have I proven K9 is working, or can it partially fail?

Anyways, I will take actual resistance measurements and publish my results.
Thanks for your help.

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1986 300E 5-Speed 240k mi.